The present invention relates generally to circuits for stabilizing black levels in the output signal of camera tubes in color television cameras, and more particularly to a circuit for stabilizing the black level in an image output signal of a camera tube irrespective of fluctuations and nonuniformity of the dark current of the camera tube in a color television camera.
In general, a dark current exists in the camera tube of a color television camera. This dark current is not continually constant, but fluctuates or becomes irregular. When there is a fluctuation in this dark current, the white balance is destroyed. Particularly in a frequency separation system in which a stripe filter is used, the green color signal transmitted at low frequency is directly influenced by the dark current.
Heretofore, color television cameras, of the single-tube or two-tube type of a color multiplex system, have used vidicon camera tubes for chrominance signals. Optical filters are provided with optically black parts which are vertical at the end portions in the horizontal scanning direction of the effective picture. A video signal obtained from a camera tube by using an optical filter of this character has a black level portion which is produced by the optically black part in the trailing edge of the horizontal beam blanking of each horizontal scanning period. Heretofore, correction for black level fluctuation due to dark current fluctuation has been carried out by clamping this black level portion, whereby the DC restoration has been carried out.
However, when there is a flaw or damage in the optically black part of the optical filter, or when there is a flaw in the photoconductive film or nesa film of the camera tube corresponding to this black part, a pulse signal of high level is generated in the signal part corresponding to the optically black part in the image pickup signal. In this known system, however, when the unwanted pulse signal is generated due to a flaw, clamping occurs with this unwanted signal as a reference, and an accurate black level clamping cannot be carried out. Furthermore, the dark current level, in general, is not uniform over the entire photoconductive surface of the camera tube in the horizontal scanning direction, but is higher at the two end parts than at the central part. (This level distribution will hereinafter be referred to as "dark current shading".) By the above mentioned known system, the effect of this dark current shading could not be reduced.
Another example of a color television camera uses an optical filter comprising a color stripe filter provided on the upper or lower portion thereof laterally with an optically black part. In this color television camera, heretofore, dark current correction has been accomplished by subjecting a signal corresponding to this optically black part in the output image pickup signal to a sampling-hold operation, thereby to detect the dark current level, generating pulses corresponding to this detected level, and applying these pulses to the image pickup signal.
However, in this known system, also, there has been the effect of the above mentioned dark current shading according to the position on the photoconductive surface of the camera tube. Furthermore, as a result of errors in the installation accuracy of an optical filter with respect to the camera tube, in general, there is a possibility of the optical filter being so installed that one portion of its optically black part projects outside of the effective scanning surface of the picture of the camera tube. In this case, in the mentioned known system, a black level signal is not produced as output in correspondence with this optically black part which is projecting outside the scanned area. For this reason, detection of and correction for the dark current level become impossible. In the above mentioned known system, moreover, the circuits, such as the sampling-hold circuit and the circuit for generating correction level depending on the hold potential, are complicated. There are other difficulties such as insufficient stability.